In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a pattern is made by irradiating a ultraviolet light to a semiconductor wafer or a glass plate for liquid crystal, but if a dust particle is sticking to a photomask used in this stage, the particle obstructs the light or refracts it, causing deformation of a transferred pattern, short-circuitry, or the like, and thus this phenomenon leads to lowering of the quality of the end products.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the photomask clean all the time; hence, in practice, the light irradiation is conducted only after a surface of the photomask is sheltered by a pellicle as a dust fender. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, and thus by setting a photo focus on a lithography pattern on the photomask, the foreign particles on the pellicle membrane fail to transfer their shadows on the photomask and thus no longer become a problem to the image transfer performance.
In general, a pellicle is built up of a pellicle frame, which is an endless frame bar usually made of aluminum, a stainless steel, or polyethylene, and a transparent pellicle membrane usually made of cellulose nitrate, cellulose acetate or a fluorine-containing polymer which transmit light well; this pellicle membrane is attached via dried solution or adhesive to one of the two annular faces (hereinafter referred to as “upper annular face”). On the other one of the two annular faces of the frame (hereinafter referred to as “lower annular face”) is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid a releasable liner (separator) for protecting the agglutinant layer.
In general, a pellicle membrane is a thin film made of a resin, so that in order to fix it on a pellicle frame in a slack and ripple-free manner, it is necessary to stretch the pellicle membrane to an appropriate extent as it is being fixed on the frame. Therefore, in the case of a rectangular pellicle, which is generally the case, the pellicle frame after being coupled with the pellicle membrane tends to curve inwardly to some extent on account of the tension of the pellicle membrane.
This phenomenon is more conspicuous in the case of a large-sized pellicle, used for manufacturing printed circuit boards or liquid crystal display panels, for example, wherein the frame side bars are relatively long, and also in the case of a small-sized pellicle, used for manufacturing semiconductor devices, wherein the frame is made of a material having low rigidity as is required for the reasons of limitations in material kind and size.
On the other hand, the photomask is required to provide as large an area as possible to be exposed to light for higher cost performance. However, the inward sagging of the frame side bars decreases the effective area of the photomask available for light exposure, and, therefore, there have been efforts to minimize the inward sagging of the pellicle frame as much as possible for better economy to thereby meet the market's expectation.
Hence, heretofore, pellicle frames have been proposed so as to solve the problem of the pellicle frame sagging, such as is described in IP Publication 1. This pellicle frame has a pair of opposing frame side bars which are outwardly bulged in a shape of an arc in their middle parts and inwardly bulged in a shape of an arc on either side of the said outward bulging and terminate as a straight line, respectively; when the designing of this configuration is appropriately balanced, the sagging amount would be controlled to be less than a certain threshold value.